This invention relates to a reflector circuit for receiving illuminating radiation and emitting corresponding amplified output radiation in response; the invention is particularly, although not exclusively, concerned with a transponder circuit for use in a pseudo passive transponder (PPT) tag.
One type of reflector circuit, namely a conventional PPT, uses a diode detector to detect incoming radiation from an interrogating source. The diode can be operated as a modulated reflector by modulating a bias potential applied thereto with an information carrying signal such that the PPT reflects incoming radiation back to the source as modulated reflected radiation. Since the conventional PPT does not incorporate an amplifier in association with the diode, the reflected radiation is of reduced amplitude with respect to the incoming radiation; this limits a useful range over which the PPT can respond for a given radiation output from the source and given minimum detection threshold level for reflected radiation received at the source. The PPT responds to incoming radiation by emitting corresponding reflected radiation with an efficiency referred to as its xe2x80x9cconversion efficiencyxe2x80x9d. This conversion efficiency is defined as a ratio of the carrier radiation power received at the PPT to sideband radiation of the reflected radiation emitted from the PPT. For the conventional PPT described above, the conversion efficiency is typically xe2x88x928 dB or less which necessitates use of radiation output power from the interrogating source of, for example, tens of milliwatts to achieve a useful operating range from the source to the PPT of a few meters.
This operating range is too short for many applications where it is, for example for safety reasons, impermissible to employ a greater radiation output power level from the source. This problem is addressed by incorporating a transmission amplifier into the PPT to amplify radiation received thereat and thereby emit more radiation back to the interrogating source. However, inclusion of the transmission amplifier into the convention PPT results in it consuming more power which then requires the PPT to employ larger batteries, to have its batteries replaced more frequently or employ a more powerful power supply. This is a major drawback when the PPT has to be compact, for example of similar size to a plastic credit card or retailing tag affixed to products.
Examples of pseudo continuous wave radar transponders, namely types of reflector circuit, are described in a UK Patent number GB 2 051 522A. One radar transponder described therein incorporates an antenna assembly and a transmission radio frequency (r.f.) amplifier for enhancing conversion efficiency of the transponder. Since it is difficult to prevent an r.f. amplifier coupled to an antenna assembly for receiving incoming radiation and emitting corresponding amplified output radiation from spontaneously oscillating, especially if the amplifier is providing in excess of +30 dB gain, the transponder additionally incorporates a delay line and associated switches controlled from a clock generator to counteract spontaneous oscillation. In operation, incoming radiation is received at the antenna assembly and converted thereat to a received signal which is then sampled by one of the switches, amplified by the amplifier, stored in the delay line for a period of time, further amplified by the amplifier before finally being emitted as reflected radiation from the antenna assembly. Inclusion of the switches, the delay line and the transmission amplifier makes the transponder more costly and complex than the conventional PPT with diode detector described above. Moreover, the switches and the amplifier consume significant power in operation. Furthermore, it is not always practicable to decrease power consumption of the transmission amplifier further without sacrificing its signal gain.
The inventor has appreciated that it is therefore desirable in a reflector circuit incorporating one or more amplifiers to enhance its conversion efficiency and arrange for the circuit to be simpler and consume less power during operation compared to prior art reflector circuits.
Thus the present invention has arisen in an endeavour to provide a reflector circuit which is capable of consuming less power during operation compared to prior art reflector and transponder circuits, and capable of employing fewer component parts.
According to the present invention, there is provided a reflector circuit for receiving illuminating radiation and emitting corresponding amplified output radiation, the circuit comprising:
an antenna assembly for receiving the illuminating radiation and providing a corresponding received signal, and
processing means for amplifying and storing a portion of the received signal for a period of time for use in generating a corresponding output signal for emission from the antenna assembly as the output radiation,
in which the processing means incorporates reflection amplifying means for amplifying the portion of the received signal.
The invention provides the advantage that the reflection amplifying means is capable of providing enhanced power gains using few component parts and consuming relatively less power compared to transmission amplifiers used in the prior art.
Advantageously, the amplifying means comprises a reflection amplifier incorporating solely a field effect transistor, namely a silicon JFET or a GaAs device, together with a feedback arrangement to operate it within a linear region of its current/voltage characteristic such that it reflects a signal received thereat with an increased magnitude. In this mode of operation, high reflection gains in excess of +30 dB are, for example, achievable for a low transistor current consumption of a few microamperes. Such a low current consumption makes it possible for the reflection amplifier to operate for many months continuously on power supplied from button cells as frequently used, for example, in electronic wrist watches. Moreover, such a low current consumption is not achievable if a transmission amplifier were used instead of the reflection amplifier. Moreover, the transmission amplifier would have a more complex circuit configuration compared to that of the reflection amplifier.
In the reflector circuit of the invention, a reflection amplifier would be expected to be unstable and spontaneously oscillate. However, by careful choice of impedance matching, reflections of signals from components directly connected to the amplifier can be arranged to be in antiphase at the amplifier, thereby counteracting spontaneous oscillation therein. This drawback is not so significant for transmission amplifiers because of their associated input-output isolation.
Since the reflection amplifying means is capable of providing a high power gain in excess of +30 dB, the reflector circuit of the invention is susceptible to spontaneous oscillation as arises in the prior art transponders and reflector circuits incorporating transmission amplification; this arises despite careful choice of impedance matching. Thus, in a preferred embodiment of the invention, the reflector circuit can incorporate gain controlling means for switching the reflection amplifying means alternately between a relatively more reflecting state and a relatively less reflecting state, thereby operable to counteract spontaneous closed-loop oscillation within the circuit. This provides the advantage that susceptibility of the circuit to spontaneous oscillation can be reduced without there being a need to incorporate signal directing switches as employed in the prior art.
In a preferred embodiment of the invention, the reflection amplifying means conveniently comprises a reflection amplifier incorporating a transistor configured by means of a feedback arrangement to operate within different portions of the transistor""s current/voltage characteristics, thereby operating the relatively more reflecting state and the relatively less reflecting state. This provides the advantage of being a relatively simple manner to provide interrupted gain within the reflection amplifying means without needing to use signal directing switches as in the prior art.
In order to counteract spontaneous oscillation within the reflector circuit, it can be operated so that the amplifying means is in a less reflecting state when re-reflected signals arising from imperfect impedance matching at the antenna assembly are reflected back to the amplifying means. The processing means may therefore advantageously employ storing means for delaying signals propagating to and from the amplifying means so that the gain controlling means is provided with sufficient time to periodically reduce gain provided by the amplifying means to counteract spontaneous oscillation within the circuit. Thus, in a preferred embodiment of the invention, the processing means advantageously incorporates storing means for storing the portion of the signal for use in generating the output signal, the storage means connected in a signal path between the reflection amplifying means and the antenna assembly.
In some situations, the reflector circuit may be simultaneously interrogated by several sources emitting radiation at mutually different frequencies, one or more of the sources emitting sufficiently strongly to cause overload, saturation or distortion within the circuit. This can prevent the circuit from responding effectively to those sources emitting radiation more weakly which do not give rise to such effects. The circuit can therefore be arranged so that the processing means is operable to provide frequency selective amplification in response to the magnitude of components present in the illuminating radiation. This provides the advantage that the processing means selectively reduces its amplification for components in the received signal which are likely to give rise to overload, saturation or distortion in the circuit.
If the reflector circuit responds non-progressively when the magnitudes of components in the illuminating radiation exceed a threshold power level at which the circuit selectively modifies its response to counteract overload or provide compression, a problem of spurious circuit response can arise. The circuit can therefore be arranged to provide amplification which progressively reduces in response to increased magnitude of components in the illuminating radiation. This provides an advantage that spurious circuit response is reduced when the magnitude of components in the illuminating radiation is substantially similar to the threshold power level.
In order to obtain a relatively compact and simple reflector circuit, it is desirable that components incorporated therein simultaneously provide a number of different functions. Therefore, in the transponder circuit, the storing means advantageously incorporates a magnetostatic wave device arranged to store the portion of the signal for use in generating the output signal and to provide the frequency selective response. This provides the advantage that the device performs two functions simultaneously.
Conveniently, the magnetostatic device provides a signal propagation path through an epitaxial Yttrium Iron Garnet magnetic film having a thickness in a range of 10 xcexcm to 100 xcexcm for storing the portion of the signal and providing a frequency selective response. This provides the advantage of being an inexpensive and compact manner to provide the selective response.
Conveniently, the antenna assembly comprises a first antenna element for receiving the illuminating radiation and a second antenna element for emitting the output radiation, said first and second antenna elements being mutually spatially separate. This provides the advantage that input and output from the circuit are isolated to a greater degree, thereby counteracting susceptibility of the circuit to oscillate spontaneously.
The antenna assembly preferably incorporates one or more of a patch antenna, a bow tie dipole antenna and a travelling wave antenna. These provide the advantage of being compact and suitable for use at radio frequencies in a frequency range of several GHz.